A broad variety of industries ranging from those engaged in chemical production to power generating plants utilize gas or fluid transporting ducts or pipes to manipulate fluid masses or material along various process locations. For example, furnace installations may be employed to burn-off poisonous or corrosive gases. For such applications, regulatory agencies may require measurement of the true amount of air, fuel, and hazardous gas entering the furnace, as well as that being removed through the stack such that the operator may carry out a mass balancing form of operation. The size of ducts and stacks carrying these gases may be quite large, for instance up to 60 inches in diameter or the rectangular equivalent thereof. Movement of gases through such ducts will be effected by any of a broad range of dynamics including fans, flow direction changing components, dampers, restrictors, and the like, and the overall gas flow across any given duct section is one exhibiting what may become a range of gas flow directions having various velocity vectors which are manifested by such phenomena as swirls, local turbulences, and the like. Generally, straight runs of such ducting with aspect ratios serving to permit flow characterizations are seldom available to permit facile mass flow measurement. Some power plant installations require that a study or correlation be carried out between gas movement and filter particulate material collection, an endeavor referred to as isokinetic measurement. To be accurate, such measurements must also be carried out with reliable evaluations of actual mass transport, even though the gas transfer at hand cannot be accurately characterized. Measurements of mass transport of gaseous materials also may be required at the commencement of a given process. For example, hoods collecting hazardous chemical vapors typically incorporate a fan and duct arrangement within which sensors must be placed to accurately measure gas flow, thus assuring at least a minimum of gas transport to protect personnel working in adjacent plant regions. Because of the dynamics of such fans and the use of flow direction changing elbows, dampers and the like, the overall gas flow across any given duct section for the hoods is one exhibiting the noted range of gas flow directions or variations of flow velocity vectors manifesting an overall flow pattern which cannot be characterized.
The technique of carrying out gas flow measurement for such applications as discussed above typically looks to the use of a series of small flow rate sensors positioned strategically across a duct cross section. The type sensor employed for such localized measurement may vary. In this regard, flow sensors are available as small turbine meters, strain bars (target meters), pitot tubes, thermal probes, and the like, all measuring flow rate by detecting local fluid speed. Because of the noted variations of fluid flow directions or velocity vectors, these localized sensors have exhibited unacceptable inaccuracies which, in turn, are manifested in inaccurate mass transport or flow averaging computations otherwise revealing true movement of material along a duct. Calibration of these flow measuring instruments or calibration checking, requires establishing a conduit system having long, straight runs upstream and downstream of the sensor or probe to establish a steady and characterized flow profile. The calibrating conduit may also be provided with flow straighteners to establish flow profile, thus reducing required run length. Flow straighteners generally are configured as a grouping of partitions (e.g. a tube bundle) arranged to break-up the duct into smaller, parallel duct channels so that the pressure drop across each channel location of the straightener is made more uniform than it may have been without the straightener. The straightener is, therefore, a flow resistor with each section having a high "aspect ratio", i.e. conduit axial direction (length) to "width" or diameter ratio, e.g. 20:1. The result of such calibration is to achieve accuracy only when the flow sensors are subjected to a characterized fluid velocity axially aligned with the conduit within which they are positioned and, correspondingly, in alignment with the flow axis of the sensor iself. Output flow signals generated by the sensors when employed in determining mass transport, can deviate strongly from that obtained during the noted calibration if the fluid flow direction incident at the localized position of the sensor is off axis with respect to the axis of the sensor. The thus-measured signal can be much greater or much less than the conduit axial component of fluid velocity which, in turn, well may lead to gross error in fluid flow measurement.
A generally unrecognized limitation of these sensors is concerned with their failure to accommodate to fluid flow velocity vectors not parallel with their axes of flow confrontation. The output signals of the devices do not respond to an expected cosine distribution, or have a flat response to speed of fluid gas flow as a function of the noted incident angle where that angle varies from the axis of the sensor. In this regard, thermal sensors generally employ a two probe configuration, one probe serving to house a temperature sensor to sense the fluid temperature providing a signal reference. The second probe is electrically heated and contains a temperature sensor in close physical contact with the heater (or such components may be combined as one element) and operates at a temperature higher than that of the reference signal probe. This heated probe provides a flow velocity sensitive signal since its temperature varies with fluid velocity, or its measured power if its structure is such as to maintain a temperature differential between the two probes. Incident flow velocity vectors varying from the axis of such a probe engender inaccuracies resulting in unreliable sensing outputs. This phenomena heretofore has been unrecognized or, at best, ignored.
The inability of individual flow sensors to respond to what is, in effect, the cosine of the velocity vector of flow impinging upon them can represent a dramatic limitation in accuracy where several such sensors are spaced in large diameter conduits to perform flow averaging. For such large diameter conduits, it is necessary to have several points in an approximate plane perpendicular to the direction of the conduit axis for carrying out measurement of local velocities. Flow averaging is particularly necessitated if the flow profile for the large duct is not characterized as "fully developed" as is generally the case. Such conditions have common causes notwithstanding duct size, including the noted flow path elbows, sideline intersections, control valves or the like. Heretofore, it has been common practice where an independent measurement shows sensed and averaged flow to be excessively high or low to simply correct the output signal of the flow sensor array to obtain agreement with the known value. This approach represents an unwise procedure more than likely leading to further unreliable measurement.
These alignment limitations of flow sensors also constitute limitations to the use of such sensors as probes to find the actual velocity vector of fluid flow in other applications. For example, it is very often desirable to map fluid flow velocity vectors in such industrial installations as very large chimneys or huge dryers having diameters of 20 feet or more. The latter devices, for instance, are designed to achieve a swirling gas motion to the extent that it is possible for gases along the outside walls of the devices to be rising while those gases at the center are moving downwardly. Often it is desirable to be able to measure the flow profile of the gases in such devices. Where the fluid flow sensors employed for this probing procedure are inaccurate for any off axis incident gas velocity vectors, their use is questionable for such flow vector mapping purposes.
Where an array of flow sensors is employed across a larger duct to measure flow, the current practice for mounting them looks to the use of threaded fittings or weldaments to mount the sensors on a single insertion rod. Very often, the thus-mounted sensors are damaged. For example, where turbine meters are employd, the delicate rotating vanes may become damaged. Similarly, the exposed probes of thermal type sensors may become bent so as to damage the temperature sensors and heater elements contained therein. In some instances, sensors of the thermal type are housed in a sequence of integrated shrouds to obtain probe protection. To carry out necessary repairs of any given one of the probe sensors, the entire array necessarily is removed from the duct installation and the process for correction becomes arduous, involved, and costly.